Tactical Release of a Sexually-Selected Pheromone in a Swordtail Fish

Fisher HS (2011) Tactical Release of a Sexually-Selected Pheromone in a Swordtail Fish. PLoS ONE 6(2): e16994. doi:10.1371/journal.pone.0016994 Tactical Release of a Sexually-Selected Pheromone in a Swordtail Fish Gil G. Rosenthal 0 Jessica N. Fitzsimmons 0 Kristina U. Woods 0 Gabriele Gerlach 0 Heidi S. Fisher 0 Daphne Soares, University of Maryland, United States of America 0 1 Department of Biology, Texas A&M University, College Station, Texas, United States of America, 2 Centro de Investigaciones Cient ficas de las Huastecas ''Aguazarca'' , Calnali, Hidalgo , Mexico , 3 Department of Biology, Boston University , Boston , Massachusetts, United States of America , 4 Marine Biological Laboratory, Woods Hole, Massachusetts , United States of America Background: Chemical communication plays a critical role in sexual selection and speciation in fishes; however, it is generally assumed that most fish pheromones are passively released since most fishes lack specialized scent glands or scent-marking behavior. Swordtails (genus Xiphophorus) are widely used in studies of female mate choice, and female response to male chemical cues is important to sexual selection, reproductive isolation, and hybridization. However, it is unclear whether females are attending to passively produced cues, or to pheromones produced in the context of communication. Methodology/Principal Findings: We used fluorescein dye injections to visualize pulsed urine release in male sheepshead swordtails, Xiphophorus birchmanni. Simultaneous-choice assays of mating preference showed that females attend to species- and sex-specific chemical cues emitted in male urine. Males urinated more frequently in the presence and proximity of an audience (conspecific females). In the wild, males preferentially courted upstream of females, facilitating transmission of pheromone cues. Conclusions/Significance: Males in a teleost fish have evolved sophisticated temporal and spatial control of pheromone release, comparable to that found in terrestrial animals. Pheromones are released specifically in a communicative context, and the timing and positioning of release favors efficient signal transmission. - Funding: This work was funded by the National Science Foundation (www.nsf.gov), IOB-0447665. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. A fundamental concern of animal communication is when and where to emit signals. While some signals, like many morphological ornaments, are always on, most signals like song, advertisement calls, and motor displays are emitted in a way that optimizes transmission to intended receivers [1]. If signals are costly to produce, or if they incur the risk of eavesdropping by rivals or predators [2] selection should favor tactical signaling. Specifically, signaling should occur preferentially in the presence of intended receivers, i. e. the audience effect, [3] and in such a way as to optimize transmission in the physical environment [4]. Such tactical signaling has contributed to the effectiveness of chemical communication across taxa. The highly complex social organization of ants, for instance, is mediated through sophisticated patterns of pheromone release in time and space [5]. In salamanders, release of male sexual pheromones in concert with stereotyped courtship motor patterns stimulates female receptivity [6]. Among aquatic animals, male crustaceans release urine towards opponents during agonistic interactions with other males [7,8]. Chemical communication is also widespread in fish [9]. While chemosignals are sometimes associated with specialized scent glands [10,11], release of chemical signals in fish is often hard to identify. However, urine-borne chemicals in fishes can serve a signal function even in the absence of derived morphological structures or visually-apparent behaviors. In goldfish (Carassius auratus), steroid conjugates released in the urine are a signal of female sexual receptivity, and females increase urination frequency in the presence of males[12]. In Mozambique tilapia (Oreochromis mossambicus), urine-borne signals indicate male social dominance [13], and males increase urination frequency in the presence of sexually receptive females [14]. Swordtails (Poeciliidae: genus Xiphophorus) are an important model system in animal communication. Numerous studies have addressed the role of male chemical cues in species recognition [15,16,17,18,19,20]. Cues are evolutionarily labile, and phylogenetically more derived signals tend to be more attractive to females [18,21]. With one exception [21], however, females prefer the scent of conspecific males over heterospecific male scent. In multimodal tests of female mating preferences, olfactory cues are more salient than sexually-dimorphic visual ornaments, and preference for conspecific scent overrides preferences for heterospecific males, even though the latter are more visually conspicuous [15,22]. Accordingly, chemical cues play a determining role in reproductive isolation between sympatric swordtails. The breakdown of species recognition via chemical cues has led to recent hybridization in natural populations of swordtails[23,24]. Females also attend to individual variation in male olfactory cues, depending on both male [25] and female [26] nutritional condition. None of these studies have addressed how and when olfactory cues are released. Given the importance of olfactory information to male mating success, selection should favor signaling strategies that maximize attractiveness to females [4]. In this study, we used laboratory experiments and field observations to study the context of pheromone release in Xiphophorus. We collected the urine of male swordtails immediately following courtship events. We showed (1) that urine-based cues are sufficient to elicit female sexual response; (2) that male urine release is dependent on both the presence of and proximity to females; and (3) that males in the wild employ courtship tactics that optimize transmission of urineborne cues. Males released urine in discrete pulses (Figure 1). Female X. birchmanni preferred diluted male X. birchmanni urine over a blank control, and failed to discriminate urine from male water [16,17,18] previously occupied by courting conspecific males (n = 15, Friedman test statistic = 9.39, p = 0.009; urine vs. blank control, Wilcoxon signed-rank (WSR) test: z = 2.10, p = 0.035; urine vs. male water, WSR test: z = 1.079, p = 0.281; Figure 2). They preferred heterospecific X. malinche male urine over conspecific urine (WSR test: z = 2.166, n = 14, p = 0.030), and urine of male conspecifics over that of female conspecifics (WSR test: z = 2.803, n = 10, p = 0.0025). Males showed a substantial increase in urination rate when in the presence of females as opposed to when females were ab (...truncated)